Covid-19, the new coronavirus that has killed nearly 1,800 people in China, causes only mild disease in four out of five people who get it, the World Health Organization has said.

“It appears that Covid-19 is not as deadly as other coronaviruses, including Sars and Mers,” said the WHO director general, Dr Tedros Adhanom Ghebreyesus, adding that officials were “starting to get a clearer picture of the outbreak”.

The conclusion comes from analysis of data from Chinese authorities relating to 44,000 cases of Covid-19 in Hubei province, where the coronavirus was first recorded.

“More than 80% of patients have mild disease and will recover, 14% have severe disease including pneumonia and shortness of breath, 5% have critical disease including respiratory failure, septic shock and multi-organ failure, and 2% of cases are fatal,” Tedros said in Geneva. “The risk of death increases the older you are.”

He said children were not suffering from Covid-19 in the same way as adults, and more research was needed to find out why. There were still gaps in understanding that he hoped the WHO’s team of international experts would be able to work towards filling.

China has reported 70,365 cases of Covid-19 infection, 2,051 of those in the past 24 hours. The new cases include those confirmed by lab tests and those diagnosed through lung CT scans read by doctors. The vast majority of Chinese cases – 94% – continue to be reported in Hubei province. China says there have been 1,772 deaths. Outside China, there have been 694 cases and four deaths.

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Tedros said there appeared to be a decline in the number of new cases in China. “This trend must be interpreted very cautiously. Trends can change.” He said the WHO would continue to work day and night to help countries prepare for the potential arrival of the coronavirus, sending testing kits and protective equipment, working with manufacturers to ensure supplies are maintained, and training health workers.

He thanked countries that had contributed towards the $675m (£520m) that the WHO has said it needs to fight the epidemic, but he added: “We have not seen the urgency in the funding that we need. We have a window of opportunity now. We need resources now to ensure countries are prepared now. We don’t know how long this window of opportunity will remain.”

The WHO’s director of emergencies, Dr Michael Ryan, said the outbreak should not be described as a pandemic, even though it has spread to 25 countries outside China. “The real issue is whether we are seeing efficient community transmission outside of China and at the present time we are not observing that,” he said.

Dr Sylvie Briand, the director of pandemic and epidemic diseases at the WHO, said the organisation was working closely with Japanese authorities regarding the Diamond Princess cruise ship docked off the coast of Japan.

“We need to make sure that we focus on our objective, our public health objective, which is to contain the virus, and not to contain the people, and making sure that we can have the right balance between the health of the population in Japan and other countries, but also the health of the people being currently on this boat,” she said.

There were still many unknowns, she said. “Measures are implemented and assessed currently and monitored on a nearly hourly basis, because we learn every day and every hour more about this disease and this virus.”

Meanwhile, experts said reports that some people may have incubated the virus for far longer than the 14-day quarantine period before falling ill were likely to be anomalies.

The China-based Global Times tweeted that one person had been diagnosed 34 days after being in Wuhan and another 94 days after contact with someone who lived in Wuhan.

Dr Michael Head, a senior research fellow in global health at the University of Southampton, said: “The last data I saw suggested 95% of people had incubation of less than 12 days. There will be the outlier, the occasional person who has a longer incubation period.” But 34 days and 94 days were extremely unlikely, he said. “Those people will have had another more recent exposure.”

He added: “Incubation period is based on best possible evidence. We are learning and gaining new data all the time so [it is] entirely possible it may be revised as the outbreak carries on.”